1. Field of the Invention
This invention relates to the non-invasive determination of the concentration of an analyte, e.g., the concentration of glucose in blood, more particularly, an article and method for improving coupling between a subject and an apparatus having a sensor employing an optical probe.
2. Discussion of the Art
U.S. Pat. No. 6,241,663 describes the benefit of applying a coupling agent, such as silicone oil or mineral oil, on both the apparatus and the surface of the tissue of a subject prior to placing them in contact with each other for optical measurements. U.S. Pat. No. 6,241,663 shows that the use of a coupling agent between an optical probe and the tissue, e.g., skin, improves thermal coupling and decreases drift in the optical signal. Decrease in drift is highly desirable, especially in the case of small changes in optical signals encountered in the non-invasive determination of the concentration of analytes. However, U.S. Pat. No. 6,241,663 does not describe a method for controlling the amount of coupling agent applied to the surface of the tissue or the distribution of the coupling agent on the surface of the tissue. Typically, the coupling agent is deposited onto the surface of the tissue by means of a syringe. The thickness of the layer of coupling agent is not quantified, nor is a method for regulating the thickness of the layer of coupling agent and for ensuring the uniformity of the layer of coupling agent described. The thickness of the layer of coupling agent will affect the transmission of both light and heat between the optical probe and the tissue, thereby consequently affecting the optical signals measured. Consequently, test-to-test variations in the optical signals may result when the coupling agent exhibits test-to-test changes in thickness or distribution or both along the interface of the tissue and the optical probe. These random variations in the optical signal may hinder the calibration of the apparatus, and, consequently, adversely affect the measurement of the concentration of a biological analyte, such as glucose.
Contact of the optical probe and the skin leads to a unidirectional change in signal as a function of time, i.e., drift, even in the absence of changes in concentration of glucose. See Bruulsema, et al., Optical Properties of Phantoms and Tissue measured in vivo from 0.9-1.3 μm using Spatially Resolved Diffuse Reflectance, SPIE Vol., 2979, 1997, 325-334 and Bruulsema, et al., Correlation between blood glucose concentration in diabetics and noninvasively measured tissue optical scattering coefficient, OPTICS LETTERS, Vol. 22, no. 3, Feb. 1, 1997, 190-192. U.S. Pat. No. 4,975,581 describes such a drift and employs the first derivative of the spectrum to minimize it. This method of compensation does not address the cause of the problem. In fact, in a spatially resolved diffuse reflectance measurement taken on the skin, the drift of signal observed by Bruulsema was so large that statistical analysis of results was precluded.
The art shows methods of addressing the optical coupling issue by providing (a) media having known optical properties or (b) refractive index coupling media between the tissue and the optical probe. U.S. Pat. No. 5,596,987 and U.S. Pat. No. 5,402,778 describe methods for measuring optical properties of tissue. In particular, U.S. Pat. No. 5,596,987 discloses a spectrophotometric system including a spectrophotometer having a light source adapted to introduce radiation at an optical input port, a detector adapted to detect radiation that has migrated through a path from the input port to an optical detection port, and a processor adapted to evaluate changes between the introduced and the detected radiation. The system also includes an optical medium of a relatively large volume, forming photon escape preventing means, having selectable scattering and absorptive properties, positioning means adapted to locate the biological tissue of interest into the migration path to create a tissue-medium optical path, and processing means adapted to determine a physiological property of the tissue based on the detected optical property of the tissue-medium optical path and the scattering or absorptive properties of the optical medium. The photon escape preventing means includes an optical medium of a selectable optical property surrounding the tissue. The selectable optical property is an absorption or scattering coefficient. The medium has at least one optical property substantially matched to the selected optical property of the tissue. In one aspect, the optical coupling system includes an optical matching fluid that is contained within a flexible, optically transparent bag and disposed partially around the tissue being monitored and the input and detection ports of the system. The optical medium may include scattering material, such as solid particles having smooth, spherical surfaces, or styrofoam. The optical medium may include a liquid having selectable absorptive or scattering properties, such as an intralipid solution. The optical coupling medium may include a pliable solid having selectable scattering or absorption properties.
U.S. Pat. Nos. 5,655,530 and 5,823,951 describe an optical method for non-invasively measuring the concentration of an analyte, particularly blood analyte in blood. The method utilizes spectrographic techniques in conjunction with an improved optical interface between a sensor probe and a skin surface or tissue surface of the body containing the blood to be analyzed. An index-matching medium is disclosed to improve the interface between the sensor probe and the surface of the skin during spectrographic analysis.
The coupling agent in U.S. Pat. No. 6,241,663 is mainly used to improve the thermal and optical contact between the optical probe of the measuring apparatus and the skin. U.S. Pat. Nos. 5,655,530 and 5,823,951 describe the benefit of applying a refractive-index matching agent, such as fluorocarbons, water-based fluid mixtures, on both the non-invasive optical measuring device and the surface of the tissue prior to placing them into contact with each other for optical measurements. U.S. Pat. No. 5,596,987 describes a method that involves immersing the tissue in an optically matching coupling fluid. However, these patents do not describe a method for controlling the amount of coupling agent required or the distribution of the coupling agent on the surface of the tissue. U.S. Pat. Nos. 5,655,530; 5,823,951; and 5,596,987 do not quantify the thickness of the layer of coupling agent or describe a method for regulating the thickness of the layer of coupling agent or ensuring the uniformity of the layer of coupling agent. Too much coupling agent will lead to slippage of the measuring device with respect to tissue (e.g., skin), and generate motion artifacts in the measurements. Too little coupling agent will lead to incomplete coverage of the tissue (e.g., skin). Either of these factors will cause high variability in the results.
Accordingly, it would be desirable to provide an article and a method for applying a uniform layer of coupling agent over the surface of the tissue. It would also be desirable to provide an article and a method for applying a uniform layer of coupling agent over the surface of the tissue, wherein the uniform layer exhibits the same thickness from application to application.